I've done the same thing with a rear wheel to replace a freewheel hub with a freehub. The flange spacing and spoke hole circle diameters were close enough that it went together with no problems. I unlaced the old wheel and just followed the original spoking pattern.

However, be sure the same is true between your current hub and the Dynohub. If the Dynohub has a significantly different flange and spoke hole circle diameter your current spokes won't work.

BTW, if the wheel is truly "all steel", including the rim, I wouldn't put any time or money into it. I like my brakes to work, even in the wet.